AFAIK, when it's mild your brain is not actually starved of glucose at all, and hypoglycemic symptoms (e.g. brain fog) are caused by your endocrine system changing hormone levels (such as cortisol) to "signal" your body that it needs glucose to increase blood sugar levels. If it gets severe enough, then the brain can run out of glucose and the person can die - but AFAIK the body prioritises the brain for fuel above all else, so it really is the last thing to go.